Use aerial down triangle while retreating or trying to interrupt your opponent's lower approaches, it's extremly safe.Once you get them above you, juggle them with up square, up triangle. You can also shut down many of their approaches and keep them pressured by using his sniper. Perfectly timed siniper shots (again only do this while in the air, you get less endling lag that way as you land) can stuff virtually any approach.
While jumping around and setting up snipers, down triangles and juggles, use side circle once in a while and also mix it up by charging your netral triangle (it has great range and is unblockable). Setup Tesla spikes diagonally in the air to make it harder for your opponent to get in, they can be a life saver. Also mix in a few side triangles in their especially as they are landing as it can catch them off guard and is relatively safe at certain ranges.
So Ratchet has one of the best camping and zoning games in the game. That's what you should be focusing on.
For kills, you have two main options. If you can get a grab (this is done most easily by reading roll habits and most bad players have this) you can set them up into a corner for an unblockable level 1 setup at the right spacing. You can also do this with neutral square (this can also be turned around mid combo). However both of these are extremely risky if you miss so don't fish for them.
You can also go for level 2s. Even if you activate this raw, it's pretty reliable as you can first press circle to slow down their speed (get them into a time bubble) and then finish them off. However it's better to do this with while really close or in a setup to ensure they can't get away.
Grounded combos are not that relevant for most characters at high level play because this game is mainly an air camp fest. I could tell you a few grounded combos but they will almost never work against good players.